A basic principle of coating is that the material itself must be properly conditioned (pretreated) prior to coating. This principle also applies to coating objects made of wood. Coating should only be performed with a thorough understanding of this principle.
Woodwork to be coated has usually been processed with a saw or an automatic planer and finished with a finishing planer or a sander. These processes often leave surface defects, such as planer scrape marks, blade cuts and rough edges. Moreover, the woodwork itself can absorb oils from being handled in people's hands and can be scratched during transport. As well, the adhesives used during the assembly process may ooze out onto the surface of the wood.
Basic material conditioning is used to remove any scratches, cuts, dirt or uneven coloration from the original wood material.
First, woodwork to be coated is wetted with water, using a sponge, to discover any defects that may not be readily apparent to the human eye. Applying a wet cloth over the dented area and then ironing can repair some surface indentations.
The basic wood material should be sanded parallel to the wood grain, using #240 - #320 sandpaper. If a portable sander is to be utilized for this operation, one that employs a back-and-forth motion should be utilized.
Bleaching can be performed to remove any stains that were not removed by sanding and to eliminate discoloration. It is recommended that at least two applications of dilute bleaching agent be provided.
After these procedures, the woodwork should be wiped with water and left to dry.
(Reply from: Asahi Sunac Corporation)
For wood species such as rosewood and pine, which contain a large amount of sap resin, a thinner should be applied to the wood surface prior to sanding. This thinner will prevent sap resin from adhering to the sandpaper.
As well, it is recommended that the appropriate grit size of sandpaper be chosen, in accordance with the wood grain (refer to the following table). Furthermore, if a relatively bright finish is desired, #180 sandpaper should be used. For woodwork made from bird's eye maple, it is recommended that a fine-grit sandpaper be used, as it will produce a smoother finish.
| Japanese Beech | Birch | Zelkova | Water Oak | Manchurian Ash | |||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Fineness | Ptn | Med | End | Ptn | Med | End | Ptn | Med | End | Ptn | Med | End | Ptn | Med | End |
| #40 | N | N | N | N | N | N | N | N | N | N | N | N | N | N | N |
| #100 | N | N | N | S | N | N | N | N | S | N | N | S | S | N | S |
| #180 | S | S | N | E | S | S | S | S | E | S | S | E | S | S | E |
| #240 | E | E | S | E | E | E | E | S | E | E | E | E | E | E | E |
| #320 | E | E | E | E | E | E | E | E | E | E | E | E | E | E | E |
| E: Easily visible, S: Somewhat visible, N: Not visible | |||||||||||||||
|
Fitness: Fineness of wood grain surface; Ptn: Pattern on the wood surface; Mod: Medullary rays; End: End grain |
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(Reply from: Asahi Sunac Corporation)
"Coating finishes" can be classified as follows:
As shown above, there are several different types of finish available. This section provides a brief explanation of two typical finishes.
In this finishing method, the wood grain (pores) is not completely filled with paint or filler.
This method of finishing is simple to perform and emphasizes the unique beauty of hardwoods. Although the finish is somewhat water repellant and will prevent the build-up of dirt, it is not completely moisture proof and dirt may eventually accumulate in the wood grain.
The open pore finishing method is utilized mainly for interior construction materials and furniture. Coating procedures for open pore finishes (example)
This method involves the filling of all pores on the wood surface, including the end grain, resulting in the creation of a thick, smooth coating film. While the aforementioned "open pore finish" places priority on the texture and quality of the base wood used, the mirror finish focuses on the texture of the coating film itself, rather than on the underlying wood material.
Any imperfections or unevenness of the wood surface will tend to become magnified with a mirror finish, therefore the material to be coated must be selected very carefully. Coating procedures for polished mirror finishes (example)
(Reply from: Asahi Sunac Corporation)
Problems that result in poor coating quality may occur during any of the following stages: during the coating operation, during the drying process and even after a long period of time has passed since coating. There are approximately 20 basic types of common coating defects. As it is not possible to describe them all in detail here, we have selected 5 typical defects to explain, as follows:
If the coating material does not adhere evenly to the coating film surface, large indentations and holes may be created here and there on the coating film.
| (Causes) | (Remedies) |
|---|---|
| The coating may contain excess silicon as an additive. | Use of appropriate additives |
| Silicon or oils may be present in the atmosphere. | Improved coating environment |
| Masking tape residue remains on the surface. | Washing off tape residue |
| Dust from sanding remains on the surface. The surface must be adequately cleaned and this problem must be brought to the operator's attention. | Adequate surface cleaning, notifying the operator |
| Dust, oil, human sweat or fingerprints may be present on the surface to be coated. | Same as above |
| Adhesives may be present or may have oozed out onto the surface. | Removal of adhesives, changing the type of adhesive used |
| Grease from within the coating booth has splattered on the surface. | Using the appropriate amounts of oil and grease |
| Water or oil have mixed into the supplied air used for spraying. | Installation and regular inspection of air cleaning system |
| The undercoating material may contain silicon. | Checking the coating process and coating materials used |
| The undercoat has not been adequately sanded. | Ensuring adequate sanding |
| Spray mist from a different type of coating material may be present. | Encouraging usage of "only one coating material per booth" |
| Use of inappropriate thinner or too great a viscosity. | Inspection of coating booth ventilation system, Use of appropriate thinner, obtaining the appropriate viscosity |
Coloration from the undercoat or substrate may bleed out and affect the overcoat color.
For (Causes) (Remedies), refer to "Guidebook of Coating Film Defects and Remedies".
Coating film does not achieve the specified delustered finish after drying.
| (Causes) | (Remedies) |
|---|---|
| Delustering agent has precipitated out of solution. | Adequate stirring. |
| Humidity is too high. | Improved room ventilation or increased room temperature. |
| Minute dust particles are present on the surface of the coated object. | Cleaner coating environment. |
| Water content is too high. | Increased drying until the appropriate water content is achieved. |
| Undercoat film is too thin. | Application of adequate amount of undercoat. |
| Either the coating material has been diluted excessively, or its viscosity is too great. | Use of coating material of the appropriate viscosity. |
| Coating film is too thick. | Ensure the correct film thickness. |
| Undercoat has not been adequately dried. | Increased drying. |
| Inappropriate type of thinner was utilized. | Use of an appropriate thinner. |
| Spray dust has adhered. | Adequate ventilation and use of greater care during gun operation. |
| Excess drying agent has been added. | Proper blending. |
The boundary between the sealer used on the undercoat and the polyester coating film rises partially, resulting in the creation of patterns due to the reflection of light.
| (Causes) | (Remedies) |
|---|---|
| Partial or poor curing of the polyester resin coating material | Mix polyester resin coating materials in the correct proportions. Check the sealer. |
| Object to be coated is at a lower temperature than the coating material. | Warm the coating object. |
| Object to be coated has high water content. Object to be coated has high sap resin content. | Reduce water content to approximately 10%. Remove sap resin. |
| Insufficient amount of sealer applied, inadequate drying and poor prevention of sap resin contamination. | Use appropriate amounts and application of coating material, depending upon the particular base material. Understand the proper method of applying coating material. |
Cracks or splits occur in the coating film.
For (Causes) (Remedies), refer to the "Guidebook of Coating Film Defects and Remedies".
(Reply from: Asahi Sunac Corporation)
There are many publications about the UV drying process available on the market. Please refer to these publications.
(Reply from: Parker Engineering Co., Ltd.)
